This invention relates to a process and apparatus for sampling the content of industrial process gas streams, in which a portion of the process gas stream is extracted, conditioned for analysis, and transported to a gas analysis device. Such a system is disclosed, for example, in U.S. Pat. No. 4,738,147.
Due to the trend in government regulations to require the measurement of gases not measured in the past, such as Cl.sub.2, ClO.sub.2, NCl, NH.sub.3 and other toxic gases, a clear need exists for a sampling and analysis system that will permit the measurement of these highly reactive gases without loss in the extraction and conditioning stages.
Many gas analysis systems are exposed to a mixture of gases. However, gases other than the gas or gases of interest may cause an erroneous response in the analysis system. In the past, dilution of the process gas stream with air has been demonstrated as a reliable way to achieve lower concentrations of the interfering gas and to eliminate many interference problems. Dilution of the process gas has also been shown to be a reliable way to make the gas more transportable through long sample lines, filters and other sample system components.
Several dilution and sampling systems have been developed that have proven to be adequate for monitoring the normal compliance gases such as SO.sub.2, NO.sub.x, TRS, CO, etc. The known dilution systems, however, experience sample loss problems when attempting to measure the more reactive gases mentioned above. The primary reason for this difficulty is that existing dilution systems force the raw process gas to flow over a filter element and then through a critical orifice before dilution occurs. This is disclosed, for example, U.S. Pat. No. 5,178,022. Forcing the raw process gas to contact the filter media at the high humidity and high concentration levels as extracted from the process causes the more reactive gases to be lost to reactions on the filter surface. In some cases, the sample may be lost in unwanted reactions with other process stream components while flowing through the sample probe before even reaching the external filter and dilution system.
A diagram of a first typical dilution probe is shown in FIG. 1. All of the system components are contained in a stainless steel pipe 10, which is inserted into the process environment. The coarse filter 12, restrictor plate 14, fine filter 16, glass orifice 18, stainless steel tube fitting 20, and eductor assembly 22 are all subject to process temperature extremes between 35.degree. and 1200.degree. F. The stability of the critical components and the ultimate dilution ratio stability are extremely difficult to control under the widely changing process conditions. Attempts have been made to control the temperature of the probe body to reduce the effects of changing process temperatures but have met with limited success due to the wide range of possible temperatures and the highly corrosive environments of the process streams.
A further problem with the existing technology is the inability to calibrate through the first or coarse filter 12. Although this filter consists of only a stainless steel screen, in most cases substantial loss may be caused by this filter if the filter is allowed to become wet or partially plugged with a reactive particulate from the process. It can be seen that the sample gas must pass through two filter elements 12 and 16, a quartz orifice assembly 18 and a stainless steel tube fitting 20 before being diluted in the eductor assembly 22. Because the fine filter is exposed to the full process temperature, polymer filters which are compatible with gases such as Cl.sub.2, ClO.sub.2 and HF cannot be used. Instead, glass fiber filters must be used, and these filters are more reactive with the process gases.
Another known dilution probe 26 is shown in FIG. 2. This probe design moves the critical flow components out of the stack environment by utilizing a long sampling pipe 28, typically 4 to 10 feet long. However, this probe still requires that the process gas pass through a filter element 30 and a critical orifice 32 before dilution. The process gas, in many cases, is cooled in passage to the filter element because the pipe 28, is not maintained at the process temperature. This cooling has been shown to cause a loss of reactive gases such as ammonia before even reaching the external filter element. Also, the long probes are a problem due to the long response times that are caused by the combination of large internal volume and low flow rates.
This probe design may also experience a problem of sample loss when used on liner stacks. Liner stacks are usually large stacks that are supported by an outer concrete stack. Sampling systems are sometimes mounted on these stacks such that the sampling probe extends from the hot stack through a cool zone between the inner and outer stack and then into the sampling system. Cooling of the gas in the probe, in this cool zone, shown as zone 34 in FIG. 2, can cause severe loss of sample due to condensation and other gas reactions inside the pipe. Attempts to heat the probe in these cool zones has had limited success, but is costly and increases the maintenance requirements.